(a) Time is relative, meaning that it depends on the observer.
Suppose
a clock is not moving. Then a person who is moving at a
fraction of the speed of light sees a clock recording
time more slowly than does a person who is at rest and
who is looking at the same clock.
(b) Length is relative.
Suppose a stick measures
one meter and is not moving. Then a person who is
moving at a fractional of the speed of light sees
the stick to be less than a meter. This is not an
optical illusion -- it is a physical effect.
(c) No object can travel faster than the speed of light.
(d) Speeds do not add.
In the familiar world of
slow-moving bodies, speeds seem to add and subtract. A baseball player
who can throw a ball at 70 miles per hour is able to throw
a ball at 120 miles per hour when standing in truck
travelling at 50 miles per hour -- when the ball is
thrown forward, the speeds of the truck and ball
add. (If the ball were thrown out the back of the truck, then
the speeds would subtract and a person on the side of the
road would see the ball moving at 20 miles per hour). But
the addition (or subtraction) of speeds fails at
high velocities. A futuristic gun that can shoot
a bullet at 70% of the speed of light when placed on
a futuristic spacecraft travelling at 50% of the speed
of light cannot fire bullets at 120% of the speed
of light; the speed of the bullet and spacecraft do not add. The bullet
ends up travelling less than the speed of light. In fact, when such
a bullet is shot forward out of the spacecraft, it travels
at 88.88...% of the speed of light (=eight-ninths
of the speed of light).
(e) The speed of light is constant.
Independent
of whether one is moving away or toward a light
source, one measures the speed to light to be
the same. Likewise, the speed of light does not depend
or whether the light source is moving. This is an extreme
example of effect (d). Instead, the motion of the
source or the observer affects the color of the light. If
the source and the observer are moving away from each
other, then the frequency of the light decreases, which
is known as a red shift because yellow light becomes
more reddish. If the source and the observer are moving
toward each other, then the frequency of the light
increases, which is known as a blue shift because
yellow light becomes more bluish.
(f) The time ordering of events is relative.
Suppose
that one observer sees event A occurring before
event B. Then it is possible that another observer who
is moving with respect to the first observer to see
event A occurring after event B.